In many processes it is desirable to have isomerization of double bonds within a given molecule. Double bond isomerization is the movement of the position of the double bond within a molecule without changing the structure of the molecule. This is different from skeletal isomerization where the structure changes (most typically representing the interchange between the iso form and the normal form). Skeletal isomerization proceeds by a completely different mechanism that double bond isomerization. Skeletal isomerization typically occurs using a promoted acidic catalyst.
There are two basic types of double bond isomerization, namely hydroisomerization and non-hydroisomerization. The former uses small quantities of hydrogen over noble metal catalysts (such as Pt or Pd) and occurs at moderate temperatures while the latter is hydrogen free and typically employs basic metal oxide catalysts at higher temperatures.
Double bond hydroisomerization at moderate temperatures is mostly used to maximize the interior olefin (2-butene for example as opposed to 1-butene) since the thermodynamic equilibrium favors the interior olefin at lower temperatures. This technology is used when there is a reaction that favors the interior olefin over the alpha olefin. Ethylenolysis of 2-butene to make propylene is such a reaction. The ethylenolysis (metathesis) reaction is 2-butene+ethylene→propylenes. Mixed normal butenes (1- and 2-butenes) are reacted to maximize the 2 butenes and thus maximize propylene. Ethylene and 1-butene do not react. If in a mixture of C4 normal olefins, 2-butene can be maximized, then the reaction to propylene will be maximized.
It is well known that double bond hydroisomerization reactions occur simultaneously with hydrogenation reactions. In many commercial applications, a feedstock with highly unsaturated molecules (acetylenics and/or dienes) is processed over a fixed bed of supported noble metal catalyst in the presence of hydrogen. For example, the reaction of butadiene over noble metal catalysts can be summarized in the reaction sequence shown below:
The primary hydrogenation reaction of butadiene plus hydrogen forms 1-butene. It proceeds rapidly over the catalysts (relative rate equivalent to 1000). In the presence of hydrogen, two reactions occur with 1-butene. One is the hydroisomerization to 2-butene (relative rate of 100). This reaction requires the presence of hydrogen to proceed but does not consume hydrogen. The other reaction is hydrogenation to normal butane (relative rate of 10). The final reaction is the hydrogenation of 2-butene directly to normal butane. This is the slowest reaction (relative rate of 1) and essentially can be neglected. Under normal conditions over noble metal catalysts, it is expected that the selectivity of 1 -butene conversion will be 90% to 2-butene and 10% to n-butane. The latter represents a loss of olefins and is undesirable.
Hydroisomerization and hydrogenation reactions are known to be carried out in fixed bed reactors. U.S. Pat. No. 3,531,545 describes a process and method for double bond isomerization consisting of mixing a hydrocarbon stream containing 1-olefins and at least one sulfur-containing compound with hydrogen, heating the mixed hydrocarbon/hydrogen stream to reaction temperatures, contacting the stream with a noble metal catalyst, and then recovering the 2-olefins as a product. The process described in this patent utilizes sulfur as an additive to reduce the hydrogenation tendency of the catalyst and thus increase hydroisomerization. Sulfur is shown to be either present in the feed, added to the feed, or added to the hydrogen stream.
It is known to employ a hydrocarbon fractionation tower in combination with a fixed bed hydrogenation reactor. In U.S. Pat. No. 6,072,091, a distillation column is used in combination with at least one hydrogenation reaction zone. The hydrogenation reaction zone is associated with the rectification section of the distillation column. More specifically, hydrocarbons are removed from the rectification section of the column to hydrogenate at least a portion of the acetylenic and diolefinic hydrocarbons contained therein. The effluent from the reaction zone is then re-introduced into the rectification section of the distillation column.
It is known to carry out a hydroisomerization reaction within a catalytic distillation tower. In U.S. Pat. No. 5,087,780 (Arganbright), a process for the isomerization of butenes in a mixed C4 hydrocarbon stream is described. A stream containing 1-butene, 2-butene, and small amounts of butadiene is fed to a catalytic distillation tower containing a Pd catalyst. A small amount of hydrogen is also fed to the tower. The 1-butene, being among the most volatile of the C4s, moves overhead while the 2-butene, being less volatile, tends to go toward the bottom of the tower. Catalyst is located in the zone with higher concentrations of 2-butene, and hydroisomerization of 2-butene to 1-butene occurs. Residual 2-butene in the bottom may be recycled to the tower. If isobutylene is part of the feed mixture, it will also go overhead with the 1-butene.
In U.S. Pat. No. 6,242,661 a process for the separation of isobutylene from normal butenes is disclosed. This process also employs a catalytic distillation process incorporating the hydroisomerization reaction. A mixture of normal and isobutylenes is fed to a tower along with a small amount of hydrogen. The tower contains a Pd catalyst located within distillation structures within the tower. In this process, the catalyst is located in the upper section of the tower in a multiplicity of catalyst beds. As the fractionation occurs, the isobutylene moves overhead. 1-Butene (also a volatile component) tends to move with isobutylene. Since the system does not employ a skeletal isomerization catalyst, the isobutylene moves through the tower unaffected. However, hydroisomerization occurs in the regions of high 1-butene and the 1-butene is converted to 2-butene. This 2-butene is less volatile and moves to the bottom of the tower. In this fashion, relatively pure isobutylene is obtained overhead since the 1-butene is reacted and moves to the bottom as 2-butene.
The above processes all produce a stream that is concentrated in 2-butene. In the ethylenolysis (metathesis) reaction of 2-butene to form propylene, it is known that isobutylene is not a desired feed component. Isobutylene and ethylene will not react. Isobutylene and 2-butene will react to form propylene and 2-methyl-2-butene. This reaction has a negative effect on the propylene selectivity of the ethylenolysis reaction and is not desirable. Thus in most cases, it is preferable to remove isobutylene from a 2-butene stream prior to reaction with ethylene.
It is known to use a catalytic distillation-deisobutylenizer (CD-DeIB) to prepare a 2-butene stream for a metathesis (ethylenolysis) reactor. Similarly to U.S. Pat. No. 6,242,661 referenced above, a CD-DeIB will remove isobutylene overhead while maximizing the flow of 2-butene out the bottoms as the 1-butene is hydroisomerized to form 2-butene. The tower typically contains alternating catalyst and fractionation structures above the feed point, and fractionation structures below the feed point. Usually there are about four catalyst sections in the tower. Hydrogen is added below the feed point in order that it is sufficiently dispersed by the time it reaches the feed point.
The CD-DeIB in this service accomplishes two functions. It hydroisomerizes the 1-butene to 2-butene to improve recovery of 2-butene and maximize the production of propylene, and also hydrogenates the small remaining amounts of butadiene after the selective hydrogenation to reduce the content of butadiene, which is a poison for the metathesis catalyst. In a CD-DeIB tower, the isobutane and isobutylene are the most volatile components and tend to go overhead in the tower. The 2-butene and the n-butane are the least volatile and tend to go to the bottom. The 1-butene and butadiene have intermediate volatility and will go up or down depending upon the operation of the tower. If the tower is designed so that the 1-butene goes up, it contacts a catalyst section and is hydroisomerized to 2-butene to the limit of the 1-butene/2-butene equilibrium in the tower. The 2-butene formed from hydroisomerization of the 1-butene tends to move downward and the remaining 1-butene continues to move upward. The fractionation sections of the tower separate the 2-butene from the 1 -butene.
The butadiene which enters the CD-DeIB is slightly less volatile than the 1-butene. Some of the butadiene moves upward where it is hydrogenated over the catalyst. The primary product of the hydrogenation is 1-butene. However, a portion of the butadiene that moves upward is “fully” hydrogenated to n-butane. This constitutes a loss of n-butenes and thus a loss of feed for a metathesis unit. Some of the butadiene moves downward with the primarily 2-butene product. This butadiene is unreacted since it does not come into contact with catalyst. Butadiene can be present in no more than very low levels in the bottoms if the 2-butene is to be fed to a metathesis unit.
U.S. Pat. No. 6,420,619 is directed to a process in which both a “back end” catalytic distillation-hydrogenation unit and a catalytic distillation deisobutylenizer are employed. This concept replaces the fixed bed selective hydrogenation units normally associated with ethylene plant fractionation systems. There are typically separate fixed bed units for the C3, C4 and C5 fractions to remove the acetylenics and diolefins to low levels prior to further processing. The system of U.S. Pat. No. 6,420,619 uses a C3 to C6 hydrocarbon feedstock from a steam cracker or FCC unit. In the “back end” CDHydro section, catalytic distillation towers are used to hydrogenate acetylenics and diolefins in the stream including butadiene, methyl acetylene and propadiene and produce a propylene product stream. The bottoms of the tower produces a C4+ stream which is then sent to a fractionation system which includes a debutanizer. The C4 overhead stream from the debutanizer is routed to a CD-DeIB where hydroisomerization occurs. In addition to the C4 feed to the debutanizer, there is a C5+ recycle from the downstream fractionation system following the metathesis unit.
Three advantages of the system disclosed in U.S. Pat. No. 6,420,619 are:                1. recycle of the C5+ stream from the metathesis unit allows for a higher recycle conversion of the butenes since the conventional system uses a C4 side draw from the de-propylenizer which is intended to recycle unconverted 2-butene back to the metathesis reactor,        2. the removal of heavies prevents buildup in the recycle stream, and        3. a catalyst can be used in the debutanizer that also can be used to selectively remove any traces of butadiene.One disadvantage of a conventional CD-DeIB system is that large quantities of catalyst must be used. Another disadvantage, as indicated above, is that in order to saturate the butadiene, the fractionation tower must be designed to push the butadiene up over the catalyst. This results in a large, costly tower with very high reflux. A third disadvantage is that when the tower bottoms is to be used as a feed stream for a metathesis unit, the quantity of isobutylene in the bottoms is required to be low, thereby resulting in very high utility costs for reboiling and condensing.        
An alternative to a CD-DeIB for obtaining a 2-butene feed steam is a system which employs a fixed bed hydroisomerization unit downstream from a selective hydrogenation unit. The selective hydrogenation unit first removes butadiene to low levels. Then the effluent C4 feed stream is fed to a second fixed bed reactor and hydrogen is introduced. In the fixed bed unit the 1-butene in the stream hydroisomerizes to 2-butene and the small amount of butadiene that remains reacts. The effluent then goes to a conventional fractionating tower where the isobutylene and isobutane are separated overhead and the 2-butene goes out the bottom where it enters a disengaging drum in which any excess hydrogen is vented. The remainder of the bottoms is used as feed for the metathesis unit. This process requires less catalyst than the CD-DeIB unit because of higher driving forces for the fixed bed. The fractionating tower can be designed to allow more isobutylene to pass into the bottoms effluent, thus saving on utilities and capital since a smaller tower can be used. The disadvantage of the fixed bed system is that the quantity of n-butenes recovered is slightly lower than when a CD-DeIB is used.
U.S. Pat. No. 6,686,510 is directed to the production of high-purity isobutylene and propylene from hydrocarbon fractions having four carbon atoms. The process disclosed in this document comprises three successive stages, namely 1) the selective hydrogenation of butadiene with isomerization of 1-butene into 2-butene up to thermodynamic equilibrium; 2) the separation by distillation into a top fraction containing isobutylene and a bottom fraction containing 2-butene and butane, and 3) the metathesis of the 2-butene fraction with ethylene to produce propylene.
Thus, various systems are known for preparing 2-butene streams for use as feed streams for a metathesis unit. It would be useful to develop a method and apparatus for the selective hydroisomerization of 1-butene to 2-butene which has improved efficiency over prior known systems.